Conventionally, in a molding machine, such as an injection molding machine, for molding a product such as a disc substrate, resin melted within a heating cylinder is charged into a cavity space in a disc-molding mold, and is then cooled and hardened in the cavity space so as to obtain a disc substrate.
For such a molding process, the above-mentioned injection molding machine includes the disc-molding mold consisting of a stationary mold assembly and a movable mold assembly; an injection apparatus for charging resin into a cavity space; and a mold clamp apparatus for bringing the movable mold assembly into contact with the stationary mold assembly and separating the movable mold assembly from the stationary mold assembly. The mold clamp apparatus is operated so as to advance and retract the movable mold assembly to thereby close, clamp, and open the disc-molding mold. When the mold is clamped, a cavity space is formed between a mirror-surface disc of the stationary mold assembly and a mirror-surface disc of the movable mold assembly.
The injection apparatus includes a heating cylinder; an injection nozzle attached to a front end of the heating cylinder; and a screw disposed in the heating cylinder so that the screw can rotate and can advance and retract.
In a metering step, the screw is rotated, whereby resin is melted and accumulated in front of the screw, and the screw is retracted accordingly. During this period, the disc-molding mold is closed and clamped. Subsequently, in an injection step, the screw is advanced, whereby the resin accumulated in front of the screw is injected from the injection nozzle and charged into the cavity space. In a cooling step, the resin in the cavity space is cooled, and hole punching is performed, whereby a disc substrate is completed. Subsequently, the mold is opened, and the disc substrate is removed.
A stamper is attached to the stationary mirror-surface disc. Upon charging of the resin into the cavity space, a fine pattern of pits formed on the stamper is transferred to the resin, whereby an uneven surface is formed as an information surface of the disc substrate.
Meanwhile, a sprue is formed in a stationary-side sprue bush, and a front end of the sprue forms a gate serving as an inlet for resin to be charged into the cavity space. Resin passes through the sprue and the gate, and enters the cavity space, where the resin flows radially outward.
Therefore, a temperature gradient is formed in the cavity space, whereby the temperature of resin increases toward the gate and decreases toward the outer peripheral edge. As a result, the degree of transfer of the pattern of the stamper increases toward the gate and decreases toward the outer peripheral edge.
In view of the above, in the stationary mirror-surface disc, the distance between a medium flow passage for temperature control and a surface of the mirror-surface disc increases from the gate toward the outer peripheral edge, whereby a gradient is imparted to the cooling capacity of the temperature control in such a manner that the cooling capacity increases toward the gate and decreases toward the outer peripheral edge, whereby generation of a temperature gradient within the cavity space is suppressed.
However, in the conventional disc-molding mold, generation of a temperature gradient within the cavity space is suppressed; however, since the outer peripheral edge portion of the cavity space is located in the vicinity of the peripheral edge of the disc-molding mold, a large quantity of heat is radiated therefrom to the outside of the disc-molding mold, whereby the outer peripheral edge portion of the cavity space is cooled excessively.
As a result, the degree of transfer locally decreases in the vicinity of the outer peripheral edge of the cavity space, and quality of disc substrates is deteriorated.
An object of the present invention is to solve the above-mentioned problems in the conventional disc-molding mold and to provide a disc-molding mold which can increase the degree of transfer of a pattern of a stamper to thereby improve quality of molded products, as well as a molded product and a molding machine.